What are Facebook's most valuable assets?
1. Attention from high-value consumers.
2. Data (behavioral, demographic and preferential) on these high-value consumers.

How should they leverage these assets to make money?

If it were me, I'd simply broker the sale of that attention and data to interested parties (advertisers that want it to sell stuff and advertisers that want to learn more about consumers). There's no shortage of either.

But here's the catch. Much like a real estate broker wouldn't sell your house with you knowing it, Facebook can't sell a consumer's attention or data without them knowing it. Everything is op-in and the user always gets a portion of the transaction.